This invention relates to a method for the offshore production of hydrocarbons using a floating vessel.
Numerous systems for producing hydrocarbons from offshore fields have been proposed and implemented since hydrocarbons were first produced from shallow water platforms early in the twentieth century. Initially,/these bottom-founded platforms facilitated production from fields generally brow the platform. More recently, industry has developed the ability to produce hydrocarbons from wells which extend outward from the region of, but do not necessarily terminate directly below, the platform. Broadly speaking however hydrocarbons produced from bottom-founded platforms derive from fields extending a limited physical area around the location of the platform, and a separate platform is usually required for production from fields which are not generally below a first platform""s location. In addition, fields which are large in areal extent may require more than one platform. Once hydrocarbons have been produced, the platforms pump the hydrocarbons into available pipeline systems to enable the hydrocarbons to be transferred to appropriate markets.
As industry expanded its exploration activities into ever-deeper water depths, industry began to use compliant production systems. Compliant production systems are not rigidly fixed to the seafloor but rather are designed to respond to the are to carry the hydrocarbons to market. As with bottom-founded platforms, once a compliant production system is installed in the location for which it was intended, production of hydrocarbons derives from fields in the near region of the installation location.
One type of commonly used compliant production system is the floating production vessel (xe2x80x9cFPVxe2x80x9d). Initially, industry used FPV""s for production, but relied on separate drilling vessels to pre-drill and workover the wells. To eliminate the cost of the separate vessel, FPV designs are now available which can drill, produce from, and workover wells; these vessels are often referred to as floating drilling production, storage and offloading vessels (FDFPSO""s). Nevertheless, several limitations of these new-generation FPV""s exist. These limitations include the complexity of incorporating the drilling system into the already complex turret system, from which the mooring system is deployed and around which the FPV weathervanes, or, in the alternative, restraining the FPV from weather-vaning and installing the drilling rig above a wellbay in the center of the vessel. The first limitation raises cost and operational complexity challenges. The second raises an operational challenge. Floating vessels should preferably retain the ability to respond to environmental forces, and for FPV""s that means retaining the ability to weathervane in response to directional changes in those forces, or limiting the application to regions with mild environments. Finally, as with both bottom-founded platforms and other compliant production systems, FPV hydrocarbon production generally derives from field locations in the general region of the FPV. FPV""s are held in place by spread mooring systems, and are confined to that location during the field""s production life. As a result, FPV""s generally suffer the same hydrocarbon production constraints that are inherent to bottom-founded platforms.
The problem faced by industry in the use of FPV""s can be further described in association with FIG. 1. Vessel 15 includes a drilling rig 16 and a combined moonpool/turret 17. Drilling riser 19 and mooring lines 18 extend from moonpool/turret 17. For the purpose of this simplified depiction of the problem faced by industry, production risers, which during production would extend from moonpool/turret 17 to template 20, are not depicted. Wells 21a, 221b, 21c, and 21d extend from template 20, which rests on seafloor 6, to field 7.
Moonpool/turret 17 allows vessel 15 to weathervane, in other words point itself towards the direction of the prevailing wind, wave, and current forces, but mooring lines 18 do not allow substantial horizontal movement and vessel 15 is therefore restricted to a central mooring location approximately above template 20. This constraint results, among other reasons, from the need to minimize the bending stresses which would be created in riser 19 if vessel 15 horizontally moved substantially away from its central mooring location at a time when riser 19 was connected to template 20. As a result, vessel 15 suffers a location constraint similar to the constraint inherent to bottom-founded platforms. In other words, the FPV suffers the constraint that all risers, whether production or drilling, must extend back to a surface location which is at the central mooring location, and which in turn is approximately above the subsea template.
The production efficiency problem that results from this constraint is exemplified by wells 21a, 21b, 21c, and 21d in FIG. 1, which extend vertical distance 11 from seafloor 6 to field 7. Each well 21 also has a lateral reach 12, which is the lateral distance between a vertical line extending through template 20 (line not depicted) and the physical location of the bottom of that well. For example, well 21 a has lateral reach 12a, and well 21b has lateral reach 12b. It is understood to those skilled in the art that in deepwater the ratio of vertical distance 11 to the lateral reach 12 for a well is typically limited to a reach ratio of approximately one to three due to operational and physical constraints on the drilling equipment and the casing pipes used in the wellbores. This limitation places production efficiency constraints on the FPV system. Specifically, hydrocarbon fields extending from a central location a horizontal distance more than three times the average of the field""s depth below the seafloor may not be fully producible from a single FPV system, and a separate production system may therefore be required to produce all recoverable reserves from the field. Furthermore, in deepwater, if the field of interest is more than one or two kilometers below the seafloor, the maximum lateral reach of the wells drilled to produce that field is generally even more limited. The reach ratios for such deepwater wells may be as low as one to one, or less. Thus, even though FPV""s are not bottom-founded, they suffer the same inherent constraint of bottom-founded platforms-remote portions of the field may not be fully producible due to the horizontal location constraints on the FPV system.
One alternative of addressing this problem is to use separate subsea wellheads, and not the single well template 20 depicted in FIG. 1. Separate wellheads would allow wells 21 to extend to field 7 from different physical locations on seafloor 6, thereby extending the ability of the overall system to extract hydrocarbons from the entire field. The problem faced by this alternative arises during the drilling and workover phases of operations. By placing separate wellheads on seafloor 6, riser 19 must connect from vessel 15, in its central mooring location, to the different physical locations of the wellheads on seafloor 6, thus again creating bending stresses in riser 19. In the alternative, the mooring system could be adjusted each time riser 19 must connect to a different wellhead location so that those stresses are minimized. Any such mooring system would require special design considerations, however, since standard mooring systems are not designed for frequent changes. In addition, it is operationally undesirable to be required to make repeated adjustments to a mooring system, due to the cost of the support vessels that are often required to make such adjustments. Finally, this alternative is undesirable because the production riser system, not depicted in FIG. 1, would then be required to have the ability to move laterally to all locations to which the FPV might need to be re-moored, and although production riser systems are often compliant in nature, such a requirement adds system design constraints which are preferably avoided.
It is apparent that a need exists for a method of conducting floating operations in which the FPV can weathervane in response to environmental forces and can perform drilling, workover and other operations over a variety of subsea wellhead locations, with each such operation carried out simultaneously with production to the FPV, and in which obtaining access to the various subsea wellhead locations does not require substantial changes to the permanent mooring system. At the same time, a method is desired which has the capability of extending the reach of production wells to remote portions of the field that is to be produced. The present invention satisfies these requirements.
This invention relates generally to a method of conducting operations from a floating vessel to an offshore field. Specifically, the method involves a floating vessel having a turret-mounted mooring element and a workstation separated from the mooring element in which the vessel is moored at a central mooring location and sequentially located above one or more drill centers associated with the central mooring location. Upon completion of operations at a first drill center the vessel is weather-vaned to the heading associated with a second drill center for operations at the second center. Production can be ongoing from the first drill center while operations are ongoing at the second center. Separation of the turret-mounted mooring element from the workstation expands the reach of the wells into the field to be produced.